The invention provides a preparation method of a nanosheet self-assembled frustum-shaped (NH4)2V3O8. The preparation method comprises the following steps of firstly, dissolving vanadium pentoxide in deionized water to prepare a suspension liquid A, afterwards adding ammonia water and ethylene glycol to the suspension liquid A, and magnetically stirring the suspension liquid until the suspension liquid is clear to obtain a solution B; secondly, loading the solution B in a hydrothermal reaction kettle, placing the hydrothermal reaction kettle in a homogeneous reaction device, and naturally cooling the homogeneous reaction device after hydrothermal reaction; and finally, washing the product with water and alcohol, collecting and drying the product to obtain the nanosheet self-assembled frustum-shaped (NH4)2V3O8. The method process is simple and is easy to control, and the prepared (NH4)2V3O8 is uniform in microcrystal chemical constitution and relatively high in purity, and shows favorable electrochemical performance when taken as a positive electrode material of a lithium ion battery. Further, by the method, a large-size device and a severe reaction condition are not needed, the raw material is low in cost and is available, the preparation cost is low, the yield is high, subsequent processing is not needed, and the method is friendly to an environment and can be suitable for mass production.